The present invention relates to a method and an apparatus for converting a level of radiant energy, ultraviolet through infrared, to a digital value.
In the measurement of radiant energy such as the spectral and intensity data measured by a spectrophotometer, it is desirable to have this information available as digital data so that it can be easily processed by computers without the errors associated with analog measurements.
In a spectrophotometer, intensity in spectral content of a light source itself or the reflectance of a particular material can be measured. Characteristically, the spectrophotometer has an entrance slit and a photo detector or photo diode array as the light measuring device.
Recently, an image sensor, the IS 32 image sensor produced by Micron Technology Inc. of Boise, Id., which is basically a dynamic random access memory chip that has been optimized for radiant energy sensitivity, has become commercially available.
The device is a dual 128.times.256 array of light sensing elements and capacitors where the charge on each capacitor is proportional to the current generated by the light sensing element. When it is desired to read out the charge level on a capacitor, the address for that element location is input to the image sensor and the charge level is then detected by a comparator to determine if the charge is greater or less than a preset threshold. The comparator output is then in either a low or a high state which defines the charge level to be less than or greater than the preset threshold. Thus, by scanning the array element by element, an output pulse train is obtained which provides a binary representation of the state of each element at the time of the scan. The charge level of each capacitor can be varied by either changing the light level impinging on the array or by changing the charging time for each capacitor.
The details of the construction of the device are hereby incorporated by reference herein.